1) Field of the Invention
The present invention relates to an improvement of non-aqueous electrolyte secondary cells, which improvement is intended to improve cycle characteristics.
2) Description of the Related Art
Today, mobile information terminals such as mobile phones, notebook personal computers, PDAs, and the like are widespread. As the driving sources for them, non-aqueous electrolyte secondary cells, which have high energy density and high capacity, are widely used.
In recent years, the positive electrode active material is filled at high density in an attempt to increase the capacity of cells. However, if the positive electrode active material is pressed with high pressure for the purpose of high-density filling, the active material particles are cracked and there appear on each surface coarse active portions, and because the active portions are easy to react with the electrolytic solution, the electrolytic solution is decomposed, resulting in generation of gas. Because of this, there have been such problems that the cell expands, and that cycle deterioration occurs resulting from a reduction in the amount of the electrolytic solution.
Techniques related to non-aqueous electrolyte secondary cells are proposed in patent documents 1 to 8.
These documents are summarized as follows.
Patent Document 1: Japanese Patent Application Publication No. 2000-12025 (Abstract, claims).
Patent Document 2: Japanese Patent Application Publication No. 2000-138063 (Abstract, claims).
Patent Document 3: Japanese Patent Application Publication No. 2000-164250 (Abstract, claims).
Patent Document 4: Japanese Patent Application Publication No. 2001-307774 (Abstract, claims).
Patent Document 5: Japanese Patent Application Publication No. 2002-231307 (Abstract, claims).
Patent Document 6: Japanese Patent Application Publication No. 2002-334718 (Abstract, claims).
Patent Document 7: Japanese Patent Application Publication No. 8-64237 (Abstract, claims).
Patent Document 8: Japanese Patent Application Publication No. 9-97626 (Abstract, claims).
The techniques involved in patent documents 1-3 are those using, as the positive electrode active materials, lithium-manganese-based oxides of spinel structure, and LiBF4. It is said that with these techniques, Mn is prevented from being decomposed from the crystal structure of the positive electrode active material, and stability at the time of operation and preservation is improved.
However, lithium-manganese-based oxides of spinel structure are problematic in that the amount of the lithium that contributes to charging and discharging is small, and thus the cell capacity is reduced and the decomposition of the electrolytic solution cannot be prevented when a highly-densely-filled positive electrode is used.
Patent document 4 is a technique that uses an electrolytic solution that contains LiPF6 and LiBF4, and it is said that with this technique, such a cell is obtained that is excellent in stability at the time of overcharging.
Patent document 5 is a technique that uses an electrolytic solution that contains LiBF4 and whose HF density is from 30-1000 ppm, and it is said that with this technique, expansion and capacity reduction encountered when the cell is preserved in the high temperature environment and in the terminal charging state are prevented.
Patent document 6 is a technique that uses an electrolytic solution that contains LiBF4 at from 30-1000 ppm, and it is said that with this technique, expansion and capacity reduction encountered when the cell is preserved under the high temperature environment and in the terminal charging state are prevented.
Patent document 7 is a technique that uses an electrolytic solution that contains LiPF6 and LiBF4, and it is said that with this technique, self-charging encountered when the cell is preserved in the charged state is prevented.
Patent document 8 is a technique that uses an electrolytic solution that contains LiBF4, and it is said that with this technique, preferable discharging characteristics are obtained under the condition of large current, and stable cell performance is obtained under the high temperature environment and under the low temperature environment.
However, the techniques involved in patent documents 4-8 are problematic in that the decomposition of the electrolytic solution cannot be prevented when a highly-densely-filled positive electrode is used.